Instruction Manual
D200126X012
2502 Controllers
June 2017
21
The area ratio of the large diaphragm to the small diaphragm is 3 to 1. A 0.8 bar (12 psig) pressure change on the small
diaphragm need only be balanced by a 0.3 bar (4 psig) change on the large diaphragm.
A change in liquid level, interface level, or density changes the buoyant force exerted on the sensor displacer, which in
turn imparts a rotary motion through the torque tube shaft. The rotary motion is applied to the controller, which uses
a nozzle, bellows, and pneumatic relay to convert the rotary motion to a standard pneumatic output signal. The
output signal is sent to a final control element. In conjunction with this control element, 2502‐249 controller‐sensors
are capable of bringing the controlled variable back to a specific control point all the time.
The following descriptions show how the relay works in conjunction with the standard proportional‐plus‐reset
controller, and how the differential relief valve construction works.
2502 Controller
As long as inflow and outflow of the vessel are equal, the beam and flapper remain motionless and allow supply
pressure to bleed through the nozzle as fast as it enters the relay through the fixed restriction. A level or density
change either raises or lowers the displacer and pivots the beam and flapper with respect to the nozzle.
Note
The relay valve is double sided to control supply on one end and exhaust on the other.
An increase in level or density with direct action, or a decrease with reverse action, moves the beam and flapper closer
to the nozzle and restricts the escape of supply pressure. This builds up the loading differential on the side of the large
diaphragm and opens the relay valve to supply pressure inflow.
On the other hand, a decrease in level or density with direct action, or an increase with reverse action, moves the beam
and flapper away from the nozzle and permits supply pressure to bleed through the nozzle faster than it can enter
through the fixed restriction. This builds up the loading differential on the side of the small diaphragm, and opens the
relay valve to exhaust loading pressure.
The three‐way proportional valve can be opened and adjusted to allow some or all of the output pressure change to
feed back to the proportional bellows in order to change the proportional band of the controller. This pushes the beam
and flapper opposite the way it is being pivoted by the torque tube shaft, counteracting the pressure change in the
nozzle and again stabilizing the relay diaphragm pressure differential. The relay valve shuts off and maintains a new
output pressure according to the change in sensed displacer position.
A wide‐open proportional valve permits feedback of all the output change and produces 100 percent proportional
response. Closing of this valve produces smaller proportional responses, since part of the output change is vented
through the valve exhaust and only the remainder is available to reposition the bellows.
The reset valve can be adjusted to channel some or all of the proportional pressure into a reset bellows that opposes
proportional bellows action. This automatically dampens the effect of any proportional overcorrection by a set
amount per time interval, as long as there is a deviation from the control point.
Figure 10 illustrates these principles at work in a direct‐acting right‐hand‐mounted construction controlling liquid
inflow to a vessel, by means of a direct‐acting diaphragm‐actuated control valve. Nozzle positions and bellows
connections would be reversed for direct action with left‐hand mounting or reverse action with right‐hand mounting.
2502F Controller with Differential Relief Valve
This construction (figure 11) has a differential relief valve used to prevent proportional pressure from exceeding reset
pressure by more than a set value, a feature useful for intermittent control applications. Proportional valve output
registers in the outer chamber of the relief valve as well as in the proportional bellows.
